There are several known techniques for securing a screw base onto a lamp outer jacket. The latter generally takes the form of a glass bulb. One conventional method is to mold a threaded area on the lower or base portion of the outer jacket glass so that a screw base can be threaded thereon. During the formation of the threads an indentation also is made near the top thereof. When the lamp is being completed, one of the in-lead wires of the lamp is pressed into a lead solder slug and the lead solder slug is inserted into the indentation. The base is threaded onto the lamp over the lead solder slug, heat is applied, and the solder joint is completed. This operation electrically connects one of the lamp in-leads to the base and mechanically holds the base onto the glass bulb of the outer jacket. A significant problem with this basing technique is that the lead solder can pose an environmental problem when the lamp is disposed of, as well as creating a manufacturing hazard during the production of the lamp.
Recently, other techniques have been proposed to alleviate the hazards in the use of the lead solder. Among these techniques are those shown in U.S. Pat. Nos. 5,381,070 and 5,521,460 (these two patents being assigned to the assignee of the instant invention) and U.S. Pat. No. 5,550,422. While all of these approaches solved the problem of lead solder usage, they introduced other concerns.
For example, the solution represented by U.S. Pat. No. 5,550,422 has the undesirable requirement that the base clip must be physically attached to the lamp by snapping it onto the bottom seal of the lamp. This provides an additional step in the manufacturing process. Further, the clip is attached to an area of the glass that is normally more fragile. The area at the bottom can have sharper angles and residual stress that make the area susceptible to glass fracture. By attaching the clip to this area, glass fractures can occur both during lamp manufacture or when the lamp is be extracted from a tight socket.
The methods shown in U.S. Pat. Nos. 5,381,070 and 5,521,460 avoid these problems by making the mechanical and electrical connection through a clip that is not attached to the bottom seal of the lamp. However, a disadvantage of these two designs is that the clip has sharp edges. These sharp edges are in contact with the glass in the area where the base is mechanically secured to the lamp. When a torque is applied to the base, for example, during extraction from a socket, the sharp edges impinge on the glass and place a high-localized pressure on the glass. This pressure can achieve levels that are sufficient to fracture the glass. This is an undesired potential failure mode because the lamp jacket can separate from the base, leaving a base with sharp glass fragments in the socket. It is also possible that such a failure mode will expose the lamp electrical circuit, thus creating the risk of an electrical shock.
Another issue occurring with the designs of U.S. Pat. Nos. 5,381,070 and 5,521,460 is that they can create an outward protrusion that extends outwardly past the base shell by more than 0.050 inches, a distance that exceeds ANSI specifications. The outward extension of the clips of these patents have a straight portion that is normally bent over the base shell during the basing operation, typically forming a radius at the clip-to-shell interface. This radius, also, can often be too large and form a protrusion that is beyond the ANSI specification.